Enhancement of the surface characteristics of carbon fibers

ABSTRACT

An improved process is provided for modifying the suface characteristics of a carbonaceous fibrous material (either amorphous carbon or graphitic carbon) and to thereby facilitate enhanced adhesion between the fibrous material and a matrix material. The fibrous material is continuously passed at a relatively rapid rate through a heating zone containing a minor quantity of gaseous molecular oxygen under conditions found suitable for bringing about the desired surface modification. Composite articles of enhanced interlaminar shear strength may be formed by incorporating the fibers modified in accordance with the present process in a resinous matrix material.

United States Patent [191 Druin et al.

[ Aug. 28, 1973 George R. Ferment, Dover; Velliyur N. P. Rao, North Plainfield, all of [73] Assignee: Celanese Corporation, New York,

[22] Filed: Aug. 20, 1970 [21] Appl. No.: 65,454

[52] U.S. Cl 106/307, 23/2091, 23/2092 [51] Int. Cl. C08h 17/08, C0811 17/10 [58] Field of Search 106/307; 23/2091,

Carbon or Graphitic Fibers, Chem. Abstracts, Vol. 71, 1969, Col. l03026(h).

Primary Examiner-James E. Poer Attorney-Thomas J. Morgan, Charles Barris and Kenneth E. Macklin [57] ABSTRACT An improved process is provided for modifying the suface characteristics of a carbonaceous fibrous material (either amorphous carbon or graphitic carbon) and to thereby facilitate enhanced adhesion between the fibrous material and a matrix material. The fibrous material is continuously passed at a relatively rapid rate through a heating zone containing a minor quantity of gaseous molecular oxygen under conditions found suitable for bringing about the desired surface modification. Composite articles of enhanced interlaminar shear strength may be formed by incorporating the fibers modified in accordance with the present process in a resinous matrix material.

15 Claims, 3 Drawing Figures PATENTEI] M162 8 I973 FIG. I

FIG 2 INVENTORS MELVIN L. DRUIN ENHANCEMENT OF THE SURFACE CHARACTERISTICS OF CARBON FIBERS BACKGROUND OF THE INVENTION In the search for high performance materials, considerable interest has been focused upon carbon fibers. The term carbon fibers is used herein in its generic sense and includes graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a predominant X-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Youngs modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density. high tensile strength, and high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for carbon fiber reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels and ablative materials for heat shields on re-entry vehicles.

In the prior art numerous materials have been proposed for use as possible matrices in which carbon fibers may be incorporated to provide reinforcement and produce a composite article. The matrix material which is selected is commonly a thermosetting resinous material and is commonly selected because of its ability to also withstand highly elevated temperatures.

While it has been possible in the past to provide carbon fibers of highly desirable strength and modulus characteristics, difficulties have arisen when one attempts to gain the full advantages of such properties in the resulting carbon fiber reinforced composite article. Such inability to capitalize upon the superior single filament properties of the reinforcing fiber has been traced to inadequate adhesion between the fiber and the matrix in the resulting composite article.

Various techniques have been proposed in the past for modifying the fiber properties of a previously formed carbon fiber in order to make possible improved adhesion when present in a composite article. See, for instance, British Pat. No. 1,180,441 to Nicholas J. Wadsworth and Willian Watt wherein it is taught to heat a carbon fiber normally within the range of 350 C. to 850 C. (e.g. 500 to 600 C.) in an oxidizing atmosphere such as air for-an appreciable period of time. Other atmospheres contemplated for use in the process include an oxygen rich atmosphere, pure oxygen, or an atmosphere containing an oxide of nitrogen from which free oxygen becomes available such as nitrous oxide and nitrogen dioxide.

It is an object of the invention to provide a continuous process for rapidly and efficiently modifying the surface characteristics of carbon fibers.

It is an object of the invention to provide an improved process for improving the ability of carbon fibers to bond to a resinous matrix material.

It is an object of the invention to provide a process for modifying the surface characteristics of carbon fibers which eliminates the need for extended treatment periods.

It is another object of the invention to provide composite articles reinforced with carbon fibers exhibiting improved interlaminar shear strength.

These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.

SUMMARY OF THE INVENTION It has been found that a process for the modification of the surface characteristics of a carbonaceous fibrous material containing at least about per cent carbon by weight comprises continuously passing a continuous length of the fibrous material through a heating zone provided at a temperature of about l,000 to 1,800 C. containing a gaseous atmosphere consisting essentially of about 0.2 to 4 per cent by volume molecular oxygen and about 96 to 99.8 per cent by volume of an inert gas for a residence time of about 5 to 60 seconds, with the mole ratio of said molecular oxygen provided in said gaseous atmosphere to that of carbon present in said carbonaceous fibrous material being at least about 0.02: 1 The resulting carbon fibers may be incorporated in a resinous matrix material to form a composite article exhibiting enhanced interlaminar shear strength.

DESCRIPTION OF THE DRAWINGS FIG 1 is a photograph made with the aid of a scanning electron microscope of a portion of graphite filament which has not undergone surface modification.

FIG. 2 is a photograph made with the aid of a scanning electron microscope of a portion of a graphite filament which has been surface modified in accordance with the present process.

FIG. 3 is a photograph made with the aid of a scanning electron microscope of a portion of a graphite filament which has undergone excessive surface modification.

DESCRIPTION OF PREFERRED EMBODIMENTS I The Starting Material The fibers which are modified in accordance with the present process are carbonaceous and contain at least about 90 per cent carbon by weight. Such carbon fibers may exhibit either an armophous carbon or a predominantly graphitic carbon X-ray difi'raction pattern. In a preferred embodiment of the process the carbonaceous fibers which undergo surface treatment contain at least about per cent carbon by weight, and at least about 99 per cent carbon by weight in a particularly preferred embodiment of the process.

The carbonaceous fibrous materials may be present as a continuous length in a variety of physical configurations provided substantial access to the fiber surface is possible during the surface modification treatment described hereafter. For instance, the carbonaceous fibrous materials may assume the configuration of a continuous length of a multifilament yarn, tape, tow, strand, cable, or similar fibrous assemblage. In a preferred embodiment of the process the carbonaceous fibrous material is one or more continuous multifilament yarn. When a plurality of multifilament yams are surface treated simultaneously, they may be continuously passed through the heating zone while in parallel and in the form of a flat ribbon.

The carbonaceous fibrous material which is treated in the present process optionally may be provided with a twist which tends to improve the handling characteristics. For instance, a twist of about 0.1 to 5 tpi, and preferably about 0.3 to 1.0 tpi, may be imparted to a multifilament yarn. Also, a false twist may be used instead of or in addition to a real twist. Alternatively, one may select continuous bundles of fibrous material which possess essentially no twist.

The carbonaceous fibers which serve as the starting material in the present process may be formed in accordance with a variety of techniques as will be apparent to those skilled in the art. For instance, organic polymeric fibrous materials which are capable of undergoing thermal stabilization may be initially stabilized by treatment in an appropriate atmosphere at a moderate temperature (e.g. 200 to 400 C.), and subsequently heated in an inert atmosphere at a more highly elevated temperature, e.g. 900 to 1000 C., or more, until a carbonaceous fibrous material is formed. If the thermally stabilized material is heated to a maximum temperature of 2,000 to 3,l00 C. (preferably 2,400 to 3,100 C.) in an inert atmosphere, substantial amounts of graphitic carbon are commonly detected in the resulting carbon fiber, otherwise the carbon fiber will commonly exhibit an essentially amorphous X-ray diffraction pattern.

The exact temperature and atmosphere utilized during the initial stabilization of an organic polymeric fibrous material commonly vary with the composition of the precursor as will be apparent to those skilled in the art. During the carbonization reaction elements present in the fibrous material other than carbon (e.g. oxygen and hydrogen) are substantially expelled. Suitable organic polymeric fibrous materials from which the fibrous material capable of undergoing carbonization may be derived include an acrylic polymer, a cellulosic polymer, a polyamide, a polybenzimidazole, polyvinyl alcohol, etc. As discussed hereafter, acrylic polymeric materials are particularly suited for use as precursors in the formation of carbonaceous'fibrous materials. Illustrative examples of suitable cellulosic materials include the natural and regenerated forms ofv cellulose, e.g. rayon. Illustrative examples of suitable polyamide materials include the aromatic polyamides, such as nylon 6T, which is formed by the condensation of hexamethylenediamine and terephthalic acid. An illustrative example of a suitable polybenzimidazole is poly-2,2- m-phenylene-S ,5 -bibenzimidazole.

A fibrous acrylic polymeric material prior to stabilization may be formed primarily of recurring acrylonitrile units. For instance, the acrylic polymer should contain not less than about 85 mole per cent of recurring acrylonitrile units with not more than about mole per cent of a monovinyl compound which is copolymerizable with acrylonitrile such as styrene, methyl acrylate, methyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like, or a plurality of such monovinyl compounds.

During the formation of a preferred carbonaceous fibrous material for use in the present process multifilament bundles of an acrylic fibrous material may be initially stabilized in an oxygen-containing atmosphere (i.e., preoxidized) on a continuous basis in accordance with the teachings of U.S. Ser. No. 749,957, filed Aug.

5, 1968, of Dagobert E. Stuetz, which is assigned to the same assignee as the present invention and is herein incorporated by reference. More specifically, the acrylic fibrous material should be either an acrylonitrile homopolymer or an acrylonitrile copolymer which contains no more than about 5 mole per cent of one or more monovinyl comonomers copolymerized with acrylonitrile. In a particularly preferred embodiment of the process the fibrous material is derived from an acrylonitrile homopolymer. The stabilized acrylic fibrous material which is preoxidized in an oxygen-containing atmosphere is black in appearance, contains a bound oxygen content of at least about 7 per cent by weight as determined by the Unterzaucher analysis, retains its original fibrous configuration essentially intact, and is nonbuming when subjected to an ordinary match flame.

In preferred techniques for forming the'starting material for the present process a stabilized acrylic fibrous material is carbonized and graphitized while passing through a temperature gradient present in a heating zone in accordance with the procedures described in commonly assigned U.S. Ser. Nos. 777,275, filed Nov. 20, 1968 of Charles M. Clarke; 17,780, filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and John P. Riggs; and 17,832, filed Mar. 9, 1970 of Charles M. Clarke, Michael J. Ram, and Arnold J. Rosenthal. Each of these disclosures is herein incorporated by reference.

In accordance with a particularly preferred carbonization and graphitization technique a continuous length of stabilized acrylic fibrous material which is non-buming when ,subjected to an ordinary match flame and derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about per cent of acrylonitrile units and up to about 15 mole per cent of one or more monovinyl units copolymerized therewith is converted to a graphitic fibrous material while preserving the original fibrous configuration essentially intact while passing through a carbonization/graphitization heating zone containing an inert gaseous atmosphere and a temperature gradient in which the fibrous material is raised within a period of about 20 to about 300 seconds from about 800 C. to a temperature of about 1,600 C. to

' forma continuous length of carbonized fibrous material, and in which the carbonized fibrous material is subsequently raised from about 1,600 C. to a maximum temperature of at least about 2,400 C. within a period of about 3 to 300 seconds where it is maintained for about 10 seconds to about 200 seconds to form a continuous length of graphitic fibrous material.

The equipment utilized to produce the heating zone used to produce the carbonaceous starting material may be varied as will be apparent to those skilled in the art. It is essential that the apparatus selected be capable of producing the required temperature while excluding the presence of an oxidizing atmosphere.

In a preferred technique the continuous length of fibrous material undergoing carbonization is heated by use of an induction furnace. In such a procedure the fibrous material may be passed in the direction of its length through a hollow graphite tube or other suscepto r which is situated within the windings of an induction coil. By varying the length of the graphite tube, the length of the induction coil, and the rate at which the fibrous material is passed through the graphite tube, many apparatus arrangements capable of producing carbonization or carbonization and graphitization may be selected. For large scale production, it is of course preferred tht relatively long tubes or susceptors be used so that the fibrous material may be passed through the same at a more rapid rate while being carbonized or carbonized and graphitized. The temperature gradient of a given apparatus may be determined by conventional optical pyrometer measurements as will be apparent to those skilled in the art. The fibrous material because of its small mass and relatively large surface area instantaneously assumes essentially the same temperature as that of the zone through which it is continuously passed.

THE SUR FACE TREATMENT The continuous length of carbonaceous fibrous material is continuously passed (e.g. in the direction of its length) through a heating zone containing a gaseous atmosphere consisting essentially of about 0.2 to 4 per cent by volume molecular oxygen (preferably 0.4 to 2.0 per cent by volume molecular oxygen) and about 96 to 99.8 per cent by volume of an inert carrier gas (preferably 98 to 99.6 per cent by volume inert carrier gas) under the conditions described in detail hereafter. Suitable inert carrier gases include nitrogen, argon, and helium, etc. The heating zone is effictively isolated from the atmosphere thereby facilitating the presence of the desired quantity of molecular oxygen within the heating zone and the elimination of appreciable extraneous addition of molecular oxygen to the heating zone.

The gaseous atmosphere (heretofore described) is provided in the heating zone at a temperature of about 1,000 to l,800 C. At temperatures much below about l,000 C. the surface treatment reaction tends to be inordinately slow, and inappropriate for continuous operation on an efficient basis. At temperatures much above about l,800 C. the surface treatment reaction becomes so rapid that it is difficult to control. If desired, a temperature gradient may be provided within the heating zone which rises to the desired surface treatment temperature. The gaseous atmosphere preferably is preheated prior to introduction into the heating zone and preferably is continuously supplied to the heating zone with a portion of the gaseous atmosphere being continuously withdrawn from the heating zone whereby off gases are effectively expelled. In a preferred embodiment of the process the gaseous atmosphere is provided at a temperature of about 1,000 to l,300 C.

The concentration of molecular oxygen and the quantity of carbonaceous fibrous material provided in the heating zone are such that the mole ratio of molecular oxygen to carbon in the carbonaceous fibrous material undergoing treatment is at least about 0.02:] (e.g. 0.02:] to 0.25:1). When the mole ratio is much below about 0.02:], then the desired surface treatment tends to be inordinately slow. When the mole ratio is much above about 0.25:], then surface overtreatment accompanied by a significant loss in single filament tenacity occurs.

The contact time during which the carbonaceous fibrous material is passed through the heating zone commonly ranges from about 5 to 60 seconds. The minimum contact time varies with the concentration of molecular oxygen in the gaseous atmosphere, the temperature of the gaseous atmosphere, and the relative molar concentrations of molecular oxygen and carbon present in the carbonaceous fibrous material within the heating zone. Generally, the higher the temperature of the gaseous atmosphere, the more rapid the surface modification. Generally the higher the concentration of molecular oxygen in the gaseous atmosphere, the more rapid the surface modification. Also it has been observed that graphitic fibrous materials of high single filament Youngs modulus (e.g. in excess of 50,000,000 psi) tend to require a slightly longer contact time for optimum results than do carbonaceous fibrous materials of a predominantly amorphous X-ray diffraction pattern which generally exhibit a lower single filament Young's modulus. Also when the carbonaceous fibrous material is provided as a relatively compact assemblage of a plurality of fibers, then longer residence times may be advantageously employed as will be apparent to those skilled in the art.

The surface modification treatment of the present process generally is terminated prior to achieving a fiber weight loss is excess of 10 per cent by weight. Greater fiber weight losses are to be avoided since such weight losses are generally indicative of an excessive surface treatment and yield no commensurate advantage. In fact, the effectiveness of the surface treatment previously achieved may actually be diminished under such circumstances. Fiber weight losses of about 0.5 to 7 per cent by weight (e.g. l or 2 per cent by weight) are commonly attained in preferred embodiments of the present process.

A particularly preferred embodiment of the present process for the modification of the surface characteristics of a carbonaceous fibrous material containing at least about per cent carbon by weight and exhibiting a predominantly graphitic X-ray diffraction pattern comprises: (a) continuously introducing a continuous length of the fibrous material into a heating zone provided at a temperature of about 1,000 to 1,800 C. containing a gaseous atmosphere consisting essentially of about 0.2 to 4 per cent by volume molecular oxygen and about 96 to 99.8 parts by volume of an inert gas, (b) continuously introducing said gaseous atmosphere into said heating zone with the mole ratio of said molecular oxygen provided in the heating zone to that of carbon present in the carbonaceous fibrous material ranging from about 0.02:] to 0.25:1, (c) continuously withdrawing a portion of the gaseous atmosphere from the heating zone, (51) continuously passing a continuous length of the carbonaceous fibrous material through the heating zone at said temperature for a residence time of about 5 to 60 seconds, and (e) continuously withdrawing the resulting continuous fibrous material from the heating zone.

The theory whereby the surface of a carbonaceous fibrous material is modified in the present process is considered complex and incapable of simple explanation. It is believed, however, that the resulting modification is attributable to a combination of physical and chemical interactions between the gaseous molecular oxygen atmosphere and the carbonaceous fibrous material. Such interaction likely includes the chemical reaction of molecular oxygen with carbon adjacent the surface of the fiber.

The surface modification imparted to the carbonaceous fibrous material through the use of the present process has been found to exhibit an appreciable life which is not diminished to any substantial degree even after the passage of 30, or more days.

The surface treatment of the present process makes possible improved adhesive bonding between the carbonaceous fibers, and a resinous matrix material. Accordingly, carbon fiber reinforced composite materials which incorporate fibers treated as heretofore described exhibit enhanced shear strength, fiexural strength, compressive strength, etc. The resinous matrix material employed in the formation of such composite materials is commonly a polar thermosetting resin such as an epoxy, a polyimide, a polyester, a phenolic, etc. The carbonaceous fibrous material is commonly provided in such resulting composite materials in either an aligned or random fashion in a concentration of about to 70 per cent by volume.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific details set forth in the examples.

EXAMPLE I A high strength-high modulus carbonaceous yarn derived from an acrylonitrile homopolymer yarn in accordance with procedures described in U.S. Ser. Nos. 749,957, filed Aug. 5, 1968, and 777,275, filed Nov. 20, 1968, was selected as the starting material. The yarn consisted of a 1600 fil bundle having a total denier of about 1,000, had a carbon content in excess of 99 nitrogen padded chamber which enclosed the surface treatment furnace. Off gases were continuously displaced and withdrawn from the heat treatment zone by the continuously introduced supply of the premixed gases. Off gases were withdrawn from the surface treatment zone primarily at the yarn exit end of the tube. The fiber weight losses which occurred during the surface treatment were less than 10 per cent and com- *monly ranged from 1 to 3 per cent.

Composite articles were next formed employing the surface modified yarn samples as a reinforcing medium in a resinous matrix. The composite articles were rectangular bars consisting of about per cent by volume of the yarn and having dimensions of wt; inch X #1 inch X 5 inches. The composite articles were formed by impregnation of the yarn in a liquid epoxy resin-hardener mixture at 50 C. followed by unidirectional layup of the required quantity of the impregnated yarn in a steel mold and compression molding of the layup for 2 hours at 93 C., and 2.5 hours at 200 C. in a heated platen press at about 100 psi pressure. The mold was cooled slowly to room temperature, and the composite article was removed from the mold cavity and cut to size for testing. The resinous matrix material used in the formation of the composite article was provided as a solventless system which contained 100 parts by weight epoxy resin and 88 parts by weight of anhydride curing agent.

The following data summarizes the surface treatment conditions employed and the properties achieved.

per cent by weight, exhibited a predominantly graphitic X-ray diffraction pattern, a single filament tenacity of about 307,000 psi (12 gpd), and a single filament Youngs modulus of about 77,000,000 psi (3000 gpd). A photograph of a filament of a substantially similar untreated yarn made with the aid of a scanning electron microscope at a magnification of 6400X is provided as FIG. 1.

Portions of the yarn were continuously unwound from bobbins and 15 ends of the yarn were continuously passed while in parallel and in the form of a fiat ribbon through a heat treatment zone provided with a temperature gradient containing an atmosphere of gaseous oxygen .and nitrogen in the concentrations'indicated.

The heat treatment zone consisted of an 18 inch Inconel tube having an inner diameter of about 1 inch which was positioned within a resistance wound mufile furnace having a length of 12 inches. Three inches of the lnconel tube protruded from each end of the mufi'le furnace. A hot zone (maximum temperature portion of gradient) having a length of about 3 inches was cenirally located in the lnconel tube through which the yarn continuously passed and was adjusted to a constant temperature of about 1,050 C.

The premixed gaseous atmosphere was continuously introduced into the lnconel tube at the yarn feed end at a rate of 25.0 S.C.F.H. (std. cu. ft. per hour). Air was excluded from the heat treatment zone by means of a The horizontal interlaminar shear strengths reported were determined by short beam testing of the carbon fiber reinforced composite according to the procedure of ASTM D2344-65T as modified for straight bar testing at a 4:1 span to depth ratio.

For comparative purposes a composite article was formed as heretofore described employing an identical carbonaceous yarn without subjecting the same to any form of surface modification. The average horizontal interlaminar shear strength of the composite article was only 3000 psi.

A photograph of a filament of the surface treated yarn of Sample B (above) made with the aid of a scanning electron microscope at a magnification of 6400X is provided as FIG. 2.

For comparative purposes a sample of the untreated yarn was passed through the heating zone containing 2.0 per cent by volume oxygen and 98.0 per cent by volume nitrogen at a rate of 10 in./min., and maintained at 1,050 C. for 15 seconds. A photograph of a filament of the yarn made with the aid of a scanning electron microscope at a magnification of 6,400X is provided at FIG. 3. The large voids present upon the surface of the fiber indicate overtreatment.

EXAMPLE [I Exampte I was repeated with the exception that the three inch hot zone of the lnconel tube was provided at a temperature of l,l0O C.

The following data summarizes the surface treatment claims appended hereto. conditions employed and the properties achieved. W lai Single filament Interlaminar Mole tenacity shear Yarn Time at ratio after surface strength of speed, Percent 1,050 C. treatment, composite, Sample in./min. O2 in seconds carbon p.s.i. p.s.i.

EXAMPLE III Example 1 was repeated with the exception that the three inch hot zone of the Inconel tube was provided 15 at a temperature of 1,260 C.

The following data summarizes the surface treatment conditions employed and the properties achieved.

1. An improved process for the modification of the surface characteristics of a carbonaceous fibrous material containing at least about 90 per cent carbon by weight so as to improve its ability to bond to a resinous matrix material comprising continuously passing in the direction of its length a continuous length of said car- Singlc filament Interlarninar Mole tenacity shear Yarn Time at ratio after surface strength of l ercent l,260 C. 01/ treatment, composite, Sump]: in./mln. O in seconds carbon p.s.i. p.s.i.

EXAMPLE IV bonaceous fibrous material through a heating zone pro- Example I was repeated with the exception that a different apparatus was employed to produce a temperature gradient having a 10 inch hot zone at a temperature of about l,700 C. 35

The premixed gaseous atmosphere was continuously introduced into the yarn feed end of the ceramic tube at a rate of 25.0 S.C.F.H. Air was excluded from the heat treatment zone by means of a nitrogen padded chamber which enclosed the heat treatment furnace.

Off gases were continuously displaced and withdrawn from the heat treatment zone by the continuously introduced supply of the premixed gases. Off gases were withdrawn from the surface treatment zone primarily at the yarn exit end of the tube.

Portions of the yarn were surface treated and formed into composites as described in Example I.

The following data summarizes the surface treatment conditions employed and the properties achieved when the hot zone of the ceramic tube was provided at l,700

vided at a temperature of about l,000 to l,800 C. to which is introduced a gaseous atmosphere consisting of about 0.2 by volume of an inert gas for a residence time of about 5 to seconds, with the mole ratio of said molecular oxygen provided in said gaseous atmosphere to that of carbon present in said carbonaceous fibrous material being at least about 0.02:1.

2. An improved process according to claim 1 wherein said carbonaceous fibrous material contains at least about 95 per cent carbon by weight.

3. An improved process according to claim 1 wherein said carbonaceous fibrous material includes a substantial quantity of graphitic carbon.

4. An improved process according to claim 1 wherein said carbonaceous fibrous material is derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mole per cent of acrylonitrile units and up to about 15 mole per cent of one or more monovinyl units copolymerized therewith.

5. An improved process according to claim 1 wherein said continuous langth of carbonaceous fibrous mate- 55 rial is one or more continuous multifilament yarn. C.

Single filament Intcrlaminar Mole tenacity shear Ynrn Time at ratio after surface strength of speed, Percent l,700 U. 0:/ treatment, composite, H nnple llL/llllll. 0' in seconds carbon p.s.i. p.s.i.

A 30 U. 8 20 0. 044 296, 900 5, 400 ll 30 4. 0 20 0. 22 235, 500 7,135

Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations are to be considered within the preview and scope of the 6. An improved process according to claim 1 wherein said inert gas is selected from the group consisting of nitrogen, argon, and helium.

7. An improved process according to claim 1 wherein said heating zone is provided at a temperature of about 1 ,000 to 1300 C. and a gaseous atmosphere consisting of about 0.4 to 2.0 per cent by volume molecular oxygen and about 98 to 99.6 per cent by volume of an inert gas is introduced therein.

8. An improved process according to claim 1 wherein said mole ratio of molecular oxygen provided in said heating zone to that of carbon present in said carbonaceous fibrous material ranges from about 0.02:] to 0.25:1.

9. An improved process for the modification of the surface characteristics of a carbonaceous fibrous material containing at least about 95 per cent carbon by weight and exhibiting a predominantly graphitic X-ray diffraction pattern so as to improve its ability to bond to a resinous matrix material comprising:

a. continuously introducing a continuous length of said carbonaceous fibrous material into a heating zone provided at a temperature of about 1,000 to 1800 C.

b. continuously introducing a gaseous atmosphere consisting of about 0.2 to 4 per cent by volume molecular oxygen and about 96 to 99.8 per cent by volume of an inert gas into said heating zone with the mole ratio of said molecular oxygen provided in said heating zone to that of carbon present in said carbonaceous fibrous material ranging from about 0.02:1 to 0.25:1,

c. continuously withdrawing a portion of the gaseous atmosphere from said heating zone,

d. continuously passing in the direction of its length a continuous length of said carbonaceous fibrous material through said heating zone at said temperature for a residence time of about 5 to 60 seconds,

and

e. continuously withdrawing the resulting continuous length of carbonaceous fibrous material from said heating zone.

10. An improved process according to claim 9 wherein said carbonaceous fibrous material contains at least about 99 per cent carbon by weight.

11. An improved process according to claim 9 wherein said carbonaceous fibrous material is derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about mole per cent of acrylonitrile units and up to about 15 mole per cent of one or more monovinyl units copolymerized therewith.

12. -An improved process according to claim 9 wherein said continuous length of carbonaceous fibrous material is one or more continuous multifilament yarn.

13. An improved process according to claim 9 wherein said heating zone is provided at a temperature of about l,000 to 1300 C. and a gaseous atmosphere consisting of about 0.4 to 2.0 per cent by volume molecular oxygen and about 98 to 99.6 per cent by volume of an inert gas is continuously introduced therein.

14. An improved process according to claim 9 wherein said inert gas is selected from the group consisting of nitrogen, argon, and helium.

15. An improved process according to claim 13 wherein said inert gas is' selected from the group consisting of nitrogen, argon, and helium.

UNITED STATES PATENT oEFicE 7 CERTIFICATE OF CORRECTION Patent 3,754,957. D d August 28, 1973 Inventor(s) MELVIN L, DRUIN et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In Column 10, claim 1,, line 33, after "0.2" insert --to 4 per cent by volume molecular oxygen and about 96 to 99.8 per cent--.

Signed and sealed this 20th day of November 1973.

(SEAL) Attest:

EDWARD I I.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents FORM PO-105O (10-69) USCOMM-DC 60376-P69 9 LLs. GOVERNMENT PRINTING OFFICE: (969 0-366-au 

2. An improved process according to claim 1 wherein said carbonaceous fibrous material contains at least about 95 per cent carbon by weight.
 3. An improved process according to claim 1 wherein said carbonaceous fibrous material includes a substantial quantity of graphitic carbon.
 4. An improved process according to claim 1 wherein said carbonaceous fibrous material is derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mole per cent of acrylonitrile units and up to about 15 mole per cent Of one or more monovinyl units copolymerized therewith.
 5. An improved process according to claim 1 wherein said continuous langth of carbonaceous fibrous material is one or more continuous multifilament yarn.
 6. An improved process according to claim 1 wherein said inert gas is selected from the group consisting of nitrogen, argon, and helium.
 7. An improved process according to claim 1 wherein said heating zone is provided at a temperature of about 1,000* to 1300* C. and a gaseous atmosphere consisting of about 0.4 to 2.0 per cent by volume molecular oxygen and about 98 to 99.6 per cent by volume of an inert gas is introduced therein.
 8. An improved process according to claim 1 wherein said mole ratio of molecular oxygen provided in said heating zone to that of carbon present in said carbonaceous fibrous material ranges from about 0.02:1 to 0.25:1.
 9. An improved process for the modification of the surface characteristics of a carbonaceous fibrous material containing at least about 95 per cent carbon by weight and exhibiting a predominantly graphitic X-ray diffraction pattern so as to improve its ability to bond to a resinous matrix material comprising: a. continuously introducing a continuous length of said carbonaceous fibrous material into a heating zone provided at a temperature of about 1,000* to 1800* C. b. continuously introducing a gaseous atmosphere consisting of about 0.2 to 4 per cent by volume molecular oxygen and about 96 to 99.8 per cent by volume of an inert gas into said heating zone with the mole ratio of said molecular oxygen provided in said heating zone to that of carbon present in said carbonaceous fibrous material ranging from about 0.02:1 to 0.25:1, c. continuously withdrawing a portion of the gaseous atmosphere from said heating zone, d. continuously passing in the direction of its length a continuous length of said carbonaceous fibrous material through said heating zone at said temperature for a residence time of about 5 to 60 seconds, and e. continuously withdrawing the resulting continuous length of carbonaceous fibrous material from said heating zone.
 10. An improved process according to claim 9 wherein said carbonaceous fibrous material contains at least about 99 per cent carbon by weight.
 11. An improved process according to claim 9 wherein said carbonaceous fibrous material is derived from an acrylic fibrous material selected from the group consisting of an acrylonitrile homopolymer and acrylonitrile copolymers which contain at least about 85 mole per cent of acrylonitrile units and up to about 15 mole per cent of one or more monovinyl units copolymerized therewith.
 12. An improved process according to claim 9 wherein said continuous length of carbonaceous fibrous material is one or more continuous multifilament yarn.
 13. An improved process according to claim 9 wherein said heating zone is provided at a temperature of about 1,000* to 1300* C. and a gaseous atmosphere consisting of about 0.4 to 2.0 per cent by volume molecular oxygen and about 98 to 99.6 per cent by volume of an inert gas is continuously introduced therein.
 14. An improved process according to claim 9 wherein said inert gas is selected from the group consisting of nitrogen, argon, and helium.
 15. An improved process according to claim 13 wherein said inert gas is selected from the group consisting of nitrogen, argon, and helium. 